1. Field of the Invention
This invention relates to polyamide-imide resins and modified aromatic polyamide-imide resins, and methods for making the same. More particularly, this invention relates to a method of making a modified aromatic polyamide-imide resin which can be applied as a coating material from solutions thereof, including solutions in conventional phenolic solvents such as cresylic acid, phenols, cresols and the like. The resin has particular usefulness as a magnet wire insulation.
2. Description of the Prior Art
Polyamide-imide resins now enjoy an evergrowing usefulness in the electrical industry as insulation coatings, such as magnet wire insulation. The rate of growth of the use of aromatic polyamide-imide resins has, however, been limited due to the extremely polar, aprotic solvents usually required for their synthesis and application. Whether such prior art resins are manufactured by conventional processes in which aromatic tricarboxylic acid anhydrides are converted into acid halides and reacted with diamines such as methylene dianaline as disclosed in U.S. Pat. No. 3,260,691 and others or made by reacting aromatic tricarboxylic acid anhydrides and aromatic diisocyanates in accordance with the applicant's own U.S. Pat. Application Ser. No. 195,502 now U.S. Pat. No. 3,790,530, or other patents (many of these patents are cited in the aforementioned U.S. patent application), polar, aprotic solvents are required. Such resins have been difficult to apply as even, consistent, smooth coatings on certain substrates because of these solvents. Particular problems have been experienced when application is to fine wire which is difficult to coat, or over base insulation materials, or substrates which may be attacked by the solvents.
In many instances, polyamide-imides are useful as topcoats over a base insulation material (see for an instance U.S. Pat. No. 3,022,200); in some instances the strong polar solvents attack the base insulation material thus downgrading the total insulation rather than upgrading the insulation as would be expected. Additionally, problems have been experienced in using such solvents with conventional magnet wire towers because of the extremely high temperatures which are necessary to expel such solvents from such coatings and their tendency to autoignite at such temperatures.
It is therefore highly desirable to provide a modified aromatic polyamide-imide resin polymerized to a useful molecular weight in solvents of moderate polarity. It is also highly desirable to provide solutions of such resins having relatively low viscosities and relatively high resin contents, and to provide solutions of such resins in conventional phenolic solvents such as cresylic acid, phenols, cresols, and the like from which the same can be applied. It is also desired that such modified polyamide-imide resins will have chemical and electrical properties similar to the completely imidized unmodified polyamide-imide resins as above-described. All of this will be further desirable where conventional phenolic solvents are used inasmuch as substantial savings in solvent cost will be experienced. Most aprotic, polar solvents are at least twice as expensive as the currently commercially available phenolic solvents which are widely used in the magnet wire industry.
The term "polyamide-imide resin" as used herein refers to that group of resins which contain a plurality of amide groups and imide groups therein. The term "unmodified polyamide-imide resins" or "non-modified polyamide-imide resins" as used herein refer to those resins which contain only amide or imide "building blocks" and can be referred to as "homopolymers". The term "modified polyamide-imide resins" as used herein refer to those resins which include groups therein which are not "building blocks" of a "homopolymer".